Tree Tolerance of Ice Cover

March-April, 2007

Ice Storm Susceptibility of Tree Species Found Growing in Urban Areas (pdf) ...

THIS IS A TRIBUTE to those readers who took the time to provide feedback to the kinds of articles they would like to read in this publication (please keep your comments coming!). Jim Pook, retired arborist from the City of Hamilton, thought an article on trees that recover quickly from ice storm damage would be quite useful given the crazy weather we sometimes have in Ontario. And I couldn’t agree more. What’s that saying about March? In like a lamb and out like a lion? Well, the first of March 2007 proved to be an oxymoron bringing spectacular wind, rain, sleet and snow to most of southern (and eastern) Ontario where more than 80,000 households were without power and road closures were abundant. But enough about us, let’s just focus on the trees.

It just so happens that there is a lot of good information and opinions on this topic and happily enough, many can be found on the internet. Hauer et al. (2006) put it simply. Tree species vary in their resistance to ice accumulation. Certain characteristics, such as weak branch junctures indicated by included bark, dead and decaying branches, a broad or unbalanced crown and fine branching and a shallow or restricted root system will also increase a tree’s susceptibility to ice storm damage. They go on to say that planting a diverse urban forest that includes trees resistant to ice storms and performing regular tree maintenance to avoid or remove structural weaknesses will reduce damage caused by severe ice storms.

It is really the strength of the branch junctures that matters most. Freezing rain and subsequent ice formation on trees can put stress on weak branches and branch junctures and cause bending and breakage, the most severe type of damage. Accumulation of ice can actually increase the branch weight of trees by a factor of 10-100 times (Hauer et al., 2006). Wet snow can have a similar effect, especially on evergreens. When you add strong winds to the equation, the outcome can be massive damage to trees that is visible in both the short and long term (something to consider when standing at a client’s property scratching your head over a post-mortem tree exam two or three years from now). Depending on the species, the height and architecture of the crown, and the overall health of the tree, tolerance to ice cover can probably be estimated. Species susceptibility models have been developed to help arborists and planners make better decisions on tree planting in urban areas.

So what kinds of trees should we avoid planting in high risk areas, such as near hydro lines and residences? How about big ones? Planting trees underneath hydro lines is usually a bad idea. Most of the power outages we experience are because of trees falling on the lines. In general, we would expect fast-growing trees like poplars, silver maples and willows to be a bad choice for these high risk areas. Trees with included bark on major stems or branches, multiple leaders and low stem unions with included bark are also a recipe for disaster (Shigo, 1991). Branch weak spots (such as those from wounds, cankers, dead twigs, etc.) are also at great risk for breakage (Shigo, 1989). Trees with strong overall branching patterns (e.g. horizontal) and conical shape (excurrent) don’t intercept as much freezing rain. This form is typical of many narrow-leaved evergreen tree species and young deciduous trees (Hauer et al., 1993). Trees with strong attachments and flexible lateral branches with smaller surface areas would be more tolerant of ice cover. Consider the following table from Hauer et al., 2006 (used with permission).

Although most trees can recover on their own from ice cover damage, there are some things you can do to help. Corrective pruning and staking will help the tree to restore good form and avoid rot and infestation due to damage. Cutting back loose bark to the edge of a wound will help facilitate healing of trunk wounds. Shaking or tapping trees to remove ice build up can do a lot of damage to bark and buds and therefore should be avoided. Trees are quite brittle when frozen, it is best to wait for the tree to thaw before any attempt is made to correct the damage. See the “additional reading” list below.

Susceptibility to ice storm damage should not be the only criteria when choosing trees for urban plantings. Pest and urban stress tolerance, soil type, water needs, fruit and leaf litter and other factors such as providing shade, air purification and habitat for wildlife should also be considered. Long term planning, regular tree maintenance, risk evaluation and species diversity are key to any successful urban tree planting project. See the next page for a list of references.

Additional ALHB in Toronto-Vaughan
The Canadian Food Inspection Agency (CFIA) has detected 16 trees infested with the Asian long-horned beetle (ALHB) within the Regulated Area in the City of Toronto through regular survey activity. The affected area is located in the Finch Avenue West and Weston Road area and is comprised predominantly of industrial and residential properties. Since infested trees can be difficult to detect, CFIA has been removing susceptible host trees (e.g. Acer) within a specific buffer zone around detection sites to help eradicate this pest. Susceptible trees within 400 metres of the infested trees on industrial and residential properties and all susceptible trees including maple, elm, willow, poplar and birch, within 10 metres from the outer edge of the ravine will be removed after Notices to Dispose have been delivered to affected property owners. Tree removal is the most effective means available to eradicate this insect. Approximately 800 trees will be removed from these properties. In addition, the number of trees to be removed from an associated ravine will be determined once surveying of the ravine has been completed.

Dormant Oil – A Reminder
Dormant oil is an effective, low toxic way to minimize overwintering populations of immature insects and mite eggs. I’ve heard great things about dormant oil helping to manage magnolia scale, euonymus scale, spruce spider mite and spruce gall adelgid. Remember, insects like to hide on the undersides of twigs and foliage. Direct the application of horticultural oils to twig undersides to maximize contact with insects and mites. As for delayed-dormant and summer applications of horticultural oil, Landscape Oil is available and can be used to manage pests on some actively growing plants (consult the label for specific recommendations).

Dormant oils provide a physical barrier which restricts both the respiration and movement of many overwintering insects (e.g. adelgids, scales, mites). To avoid foliar damage, dormant oil is best used when min/max temperatures remain somewhere between 5 and 15oC during the first week or so after application. This is especially important on evergreens since their foliage is always present and always susceptible to injury. Try to make applications when conditions will facilitate rapid drying of treated twigs. This will also help minimize phytotoxicity problems. Avoid mixing oil solutions with sulphur and other fungicides as plant damage may result.

Always read the label carefully since many plant species are sensitive to dormant oils (see below). Avoid spraying red delicious, mutsu or empire cultivars of Malus as bark injury may result (I’ve actually seen this). Do not apply to Malus or Pyrus after green tip. I’ve had professionals tell me that they have used dormant oil on Taxus and Thuja for years and never had a problem. Others have seen quite a bit of burn on those two genera – yikes. Remember, oil and water do not mix. You will need to agitate the mixture constantly in order to ensure even coverage. If over application occurs and extreme temperatures are experienced within a week or two, damage may result. Applications should be made during mild mornings when no rain is in the forecast to facilitate drying.

Plants Sensitive to Dormant Oil Treatments: Acer palmatum (Japanese maple), Acer rubrum (red maple), Acer saccharum (sugar maple), Carya (hickory), Cryptomeria, Juglans sp. (walnut), Juniperus (blue cultivar selections), Picea pungens glauca (blue Colerado spruce), Pinus strobus (white pine), Quercus rubra (red oak) Taxus (yew) and Thuja (cedar).

Other, Less-Sensitive Plants to Dormant Oil Treatments: Cercis canadensis (redbud), Fagus (beech), Ilex crenata (Japanese holly), Picea abies (Norway spruce), Picea glauca (white spruce), Pseudotsuga menziesii (Douglas fir). ©

Ice Damage References
• Hauer, R.J., Dawson, J.O. and Werner, L.P. 2006. Trees and Ice Storms: The Development of Ice Storm-Resistant Urban Tree Populations, 2nd Edition. Joint Publication 06-1, College of Natural Resources, University of Wisconsin-Stevens Point, and the Department of Natural Resources and Environmental Sciences and the Office of Continuing Education, University of Illinois at Urbana-Champaign. 20 pp.

• Hauer, R.J., Wang, W. and Dawson, J.O. 1993. Ice storm damage to urban trees. Journal of Arboriculture 19:187-193.

• Shigo, A.L. 1991. Modern Arboriculture. Shigo and Trees, Associates Durham , New Hampshire 03824-3105, U.S.A.

• Shigo, A.L. 1989. Branch failure: a closer look at crack drying. Journal of Arboriculture. 15:11-12.

Additional Reading
• Canadian Forest Service. 2006. Some Tips on How to Care For Damaged Trees. (

• Harris, R.W., Clark, J.R. and Natheny N.P. 2004. Arboriculture: Integrated Management of Landscape Trees, Shrubs and Vines. Fourth Edition. Prentice Hall.

• Landowner Resource Centre and the Ontario Ministry of Natural Resources. 1999. Extension Notes. Caring For Ice-Damaged Trees. (

• University of Maine Cooperative Extension. 2006. Recovery of Ice-Storm Damaged Trees. Leaflet #9013. (

Contact Information

This column is written by Jen Llewellyn, Ontario Ministry of Agriculture and Food and Rural Affairs (OMAFRA) Nursery Crops Specialist

Email any questions you have directly to Jen and we'll publish her response.

P: 519-824-4120 ext. 52671 • F: 519-767-0755

OMAF website:

Nursery Landscape Agriphone: 1-888-290-4441

Our mission is to enhance and promote the care and benefit of trees for present and future generations in Ontario through education, research and awareness.